First verify the correct tire pressures. Isn't the spec 35 psi on these? Reference the placard on the driver's door.
Following that attempt to train all of the sensors. The tool can be had over the parts counter at your local Ford store - I'll provide the tool # off of mine when I go to work tomorrow. My cost was $25, yours will likely be in the $40 range as mentioned.
The training procedure is key on 3 times quickly, press the brake pedal once, key on 3 times quickly and finish by leaving the key on. This has sequence must be completed in approx 8 seconds.
Start by training the l/front, then r/front, r/rear and finally l/rear. The horn will honk each time a sensor trains, and after successful training of all four tires the cluster message centre will read 'training complete'.
If one tire will not train move to the next tire. Then go back to the suspect sensor and retry. If it still will not train then you have your answer.
2003 GMC Sierra 3500 -- dually, 4x4, hasn't blown up yet and that's what i look for in a truck
Ford Doctor -- They come in broken and leave fixed. Ford of Canada Master Tech